My point is, this doesn't involve anyone who's not in their family. At all. In the slightest. If I decided to cut off all relations with my sister it wouldn't affect my relationships with my friends, or her relationships with her friends at all. In fact it would be none of their business.
So? Does that mean Dora can't tell the people who are actually closest to her in her life about a major life decision?
Some friends are the family we choose, and we tell them everything going on in our lives, including family matters. Especially when those family matters affect our mutual circle of friends (in this case, Sven's dealings with their mutual close friend Faye).
I mean sure I might want to talk it over with them for advice, get some emotional support. But it wouldn't impact their lives directly. At all. It would be a private family matter. And her exes? They would give even less of a shit.
So Marten is just an ex? Marten and Dora were friends before their relationship, and that friendship was strong enough to survive a bad breakup. Who else is Dora going to get emotional support from (which is exactly what Dora is doing here)? Tai? Their relationship is still new and tentative, and its Dora's first commitment after her breakup with Marten. Faye? She's cutting off ties with her brother partially because of how he's treated Faye in the past year (yes, a year; remember that time in the comic moves extremely slow compared to real time). Hanners? They've hung out, but they've never been all that close. Penny? Collette? They're just employees. Jim? Same situation as Tai, except even less so.
Marten's a close friend, and has remained so even after their romantic relationship fell apart. He's exactly the kind of friend you share this information with. He's also proven to be a good listener with people he cares about, no matter how remotely.
Now I know Dora is a drama queen who thinks the world revolves around her issues, and Marten is a spineless milksop who treats anything said by a person with boobs with deadly earnestness. But in actuality if Dora stopped talking to her brother it wouldn't impact anyone but their immediate family. None of the core cast even deals with Sven on a regular basis or has him as an integral part of their life.
Until Sven decides to become one, like he's done with Faye. Dora's and Marten's closest friend.
Hell, when was the last time before this new Faye thing that Dora and Sven even shared a strip together? Months? Years? Have they seemed upset by this? Has anyone noticed at all?
People noticed, kinda like how they notice with other minor characters who have an affect on the characters' lives, like Marten's mother (hey, another family member!)
Shortly after Angus and Faye got together, Sven took Hanners out on a pretend date back in 1755 to 1770. Dora wasn't thrilled (and neither was Faye), and told him point black. During that story, Sven realized he wasn't over Faye yet, but couldn't understand why.
Even if they haven't shared a strip together that much since the Sven-Faye storyline, many of Dora's issues come out of her rocky lifelong relationship with her brother, which she admitted to her therapist in 2170. She went to talk to him about it in 2192, when she found out that Sven hadn't had sex for months and was starting to reevaluate himself. (The interns were introduced just a few strips later, to put it in story context.)
Since then, Sven laid low by his own choice, moping around. That's why he hasn't been around. In the meantime, Dora, Faye, Marten, and the others moved on with their lives. Now, after his own imposed hermititude, Sven's decided that he loves Faye, and in his self-centered way, bulldozes his way into Faye's life. And thus everybody else's again.
Yes, it's been a long time since we've seen Sven in an active storyline. But that's the kind of character he is. Just because they're not actively in the strip doesn't mean that they still don't have an affect on the other characters, especially when the backstory was actually told in the strip, and it also doesn't mean they can't simply slip back into the story when driven by their own decisions. People only completely disappear when they move (like Steve's first girlfriend) or get eaten by allosaurus. That's what happens when you have an ensemble strip that's been running for more than a decade. Sven will always be part of Dora's life, no matter how much she tries to cut him out, and he's got two strong connections to two of the three major original characters.